Electronic component and electronic device

ABSTRACT

A surface of a connection terminal of an electronic component is covered with a protection layer made of a AgSn alloy. The electronic component is soldered to a connection terminal of a circuit board.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2011-282725, filed on Dec. 26,2011, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are related to an electronic componentand an electronic device.

BACKGROUND

The number of connection terminals in a semiconductor device (an LSI: alarge scale integrated circuit) has tended to increase in recent yearsalong with advances in performance and integration of such asemiconductor device, and there is a growing demand for furtherreduction in size of connection terminals.

In flip-chip mounting, a connection terminal of a semiconductor deviceand a connection terminal of a wiring board are connected to each otherby using a solder bump. Such a solder bump is made of an alloy (solder)such as Sn-3.5 wt % Ag, Sn-0.7 wt % Cu or Sn-3 wt % Ag-0.5 wt % Cu. Inthe meantime, the connection terminals of the semiconductor device andthe wiring board are usually made of Cu (copper). Surfaces of theconnection terminals may occasionally be plated with Ni (nickel) or Au(gold) in order to prevent corrosion or to improve solder wettability onthe surfaces of the terminals.

[Patent Document 1] Japanese Laid-open Patent Publication No. 10-41621

SUMMARY

According to a first aspect of the disclosed techniques, an electroniccomponent includes a connection terminal to be soldered to a differentelectronic component, in which a surface of the connection terminal iscovered with a protection layer made of a AgSn alloy.

According to a second aspect of the disclosed techniques, an electronicdevice includes an electronic component; a circuit board having theelectronic component mounted thereon; and solder bonding a connectionterminal of the electronic component to a connection terminal of thecircuit board, in which a surface of at least one of the connectionterminal of the electronic component and the connection terminal of thecircuit board is covered with a protection layer made of a AgSn alloy.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are views depicting results of investigating Cudistribution with time in the case of applying a current at a currentdensity of 6×10⁻³ A/cm² between connection terminals made of Cu under atemperature environment of 150° C.;

FIGS. 2A and 2B are views depicting results of investigating Nidistribution with time under similar conditions to those used in FIGS.1A and 1B with surfaces of the connection terminals plated with Ni;

FIG. 3A is a view depicting an example of an electronic componentaccording to an embodiment while FIG. 3B is an enlarged view depicting aportion of a connection terminal of the electronic component;

FIG. 4 is a view depicting a junction between a connection terminal ofthe electronic component and a connection terminal of a circuit board;

FIGS. 5A to 5C are views schematically depicting samples of Example andComparative Examples 1 and 2;

FIG. 6 is a graph illustrating results of investigation of time to causewiring disconnections in the samples of Example and Comparative examples1 and 2; and

FIG. 7 is a graph illustrating results of investigation of relationsbetween Ag contents in AgSn alloys and time to cause wiringdisconnections.

DESCRIPTION OF EMBODIMENTS

Prior to description of embodiments, a prelude will be given below inorder to facilitate understanding of the embodiments.

As described previously, in recent years, connection terminals ofelectronic components such as semiconductor devices have tended to bereduced in size and thus densities of currents (current densities)flowing through the connection terminals have tended to be increased.However, when a density of a current flowing through a connectionterminal becomes equal to or above about 10⁴ A/cm², electromigrationoccurs at a junction between the connection terminal and solder, therebyincreasing a resistance value between connection terminals. In anextreme case, such electromigration may result in a wiringdisconnection.

FIGS. 1A and 1B are views depicting results of investigating Cudistribution with time in the case of bonding connection terminals madeof Cu to each other by using a solder bump and applying a current at acurrent density of 6×10⁻³ A/cm² between the connection terminals under atemperature environment of 150° C. Here, a direct current flows from theupper connection terminal to the lower connection terminal. Note that anelectron probe microanalyzer (EPMA) is used for acquisition of the Cudistribution.

As apparent from FIGS. 1A and 1B, Cu atoms contained in the connectionterminals migrate from below (a low-potential side) to above (ahigh-potential side) with time. A position after migration of a Cu atomconstitutes an atomic vacancy. When a number of such atomic vacanciesare generated, such vacancies may be collectively recognized aselectromigration.

FIGS. 2A and 2B are views depicting results of investigating Nidistribution with time under similar conditions to those used in FIGS.1A and 1B with surfaces of the connection terminals plated with Ni. Asapparent from FIGS. 2A and 2B, Ni atoms migrate from below to above withtime. Therefore, Ni plating does not prevent the electromigration.

A conceivable option to suppress such electromigration is to increasethe number of connection terminals and to thereby reduce a currentdensity in each of the connection terminals. However, this leads to anincrease in a layout space for the connection terminals, which resultsin an increase in size of a semiconductor device.

In view of the above, an object of the embodiments is to provide anelectronic component and an electronic device, which are less likely tocause electromigration even when a current having a high current densityflows through a junction between a connection terminal and solder.

Embodiment

FIG. 3A is a view depicting an example of an electronic componentaccording to an embodiment and

FIG. 3B is an enlarged view depicting a portion of a connection terminalof the electronic component. Meanwhile, FIG. 4 is a view depicting ajunction between a connection terminal of the electronic component and aconnection terminal of a circuit board.

An electronic component 10 according to the embodiment includes asemiconductor chip 11 a provided with a given electronic circuit, and apackage (sealing resin) lib which seals the semiconductor chip 11 a.Meanwhile, a number of connection terminals 12 are provided on a lowersurface side of the semiconductor chip 11 a. The connection terminals 12are made of Cu and a protection layer 13 made of a AgSn (silver-tin)alloy is provided on each surface of the connection terminals 12.

Here, chemically stable AgSn is preferably used for the AgSn alloy toform the protection layer 13.

A thickness of the protection layer 13 is preferably from 3 μm to 100μm. If the thickness of the protection layer 13 falls below 3 μm, such aprotection layer 13 may contain a pin hole. This makes it difficult tocompletely cover a Cu electrode and thus Cu electromigration is notprevented sufficiently. On the other hand, if the thickness of theprotection layer 13 exceeds 100 μm, such a protection layer 13 mayreduce electric conductivity and adversely affect the electroniccircuit.

Meanwhile, a circuit board 20 includes wiring (not depicted) formed intoa given pattern, and connection terminals 22 (see FIG. 4). Eachconnection terminal 22 of the circuit board 20 is also made of Cu and aprotection layer 23 made of an AgSn alloy is provided on a surfacethereof. In addition, the connection terminal 12 of the electroniccomponent 10 is bonded to the connection terminal 22 of the circuitboard 20 by using solder 25. The solder 25 is made of a Sn-3.5 wt % Agalloy, for example.

In this embodiment, the surfaces of the connection terminals 12 and 22are covered with the protection layers 13 and 23 each made of a AgSnalloy as described above. Thus, electromigration may be suppressed.Reasons why the prevention is available will be described below.

Electromigration has a relation with a diffusion coefficient. To be moreprecise, an element having a greater diffusion coefficient is morelikely to cause electromigration. The diffusion coefficient of Cu in Snis equal to 2.4×10⁻¹¹ m²/s while the diffusion coefficient of Ni in Snis equal to 5.4×10⁻¹³ m²/s at 160° C. On the other hand, the diffusioncoefficient of Ag in Sn is equal to 9.0×10⁻¹⁵ m²/s, which is smaller byfour digits than that of Cu and by two digits than that of Ni. In otherwords, Ag in Sn is less likely to migrate upon application of ahigh-density current and less likely to generate atomic vacancies, whichlead to electromigration, as compared to Cu or Ni.

Meanwhile, Sn in Ag has the diffusion coefficient which is almost thesame as that of Ag. Moreover, since Sn is solid-solved in Ag in atemperature range equal to or below 160° C. For these reasons, an atomicvacancy generated by migration of Ag attributed to the current flowingbetween the contact terminals is buried with Sn. Accordingly,electromigration is even less likely to occur in this case.

As described above, in this embodiment, the surfaces of the connectionterminals 12 and 22 are covered with the protection layers 13 and 23each made of the AgSn alloy. Accordingly, even when the current flowingbetween the connection terminals 12 and 22 has a high current density,migration of the Cu atoms in the connection terminals 12 and 22 andmigration of the Ag atoms in the protection layers 13 and 23 aresuppressed, whereby electromigration is less likely to occur. In thisway, problems such as an increase in a resistance value attributed toelectromigration and occurrence of a wiring disconnection are avoided.As a consequence, the embodiment has an effect of improving reliabilityof a junction between the electronic component 10 and the circuit board20.

As apparent from FIGS. 1A, 1B, 2A, and 2B, migration of the atomsattributed to electromigration occurs in the direction from thelow-potential side (a cathode) to the high-potential side (an anode).Accordingly, the protection layer may only be provided on the connectionterminals on the low-potential side without providing the protectionlayer on the connection terminals on the high-potential side.

Moreover, the solder 25 connecting the connection terminals 12 and 22 isnot limited to the above-mentioned Sn-3.5 wt % Ag alloy but variousother alloys (solder) including a Sn-0.7 wt % Cu alloy, a Sn-3 wt %Ag-0.5 wt % Cu alloy may be also applied.

Nevertheless, the solder 25 connecting between the connection terminals12 and 22 is preferably made of an SnAg alloy containing Ag in a rangefrom 2.0 wt % to 4.0 wt %. When the above-described SnAg alloy is usedas the solder, this alloy has an effect of suppressing diffusion of theAgSn alloy in the protection layers 13 and 23 into the solder. Thus, awiring disconnection between the connection terminals 12 and 22attributable to electromigration may be suppressed more reliably.

Experiment 1

Some electronic components and circuit boards are soldered in accordancewith the above-described method and then time to cause a wiringdisconnection due to electromigration is investigated for each of thecombinations of the electronic components and the circuit boards.Results are described below.

A pair of copper patterns each having a width of 100 μm and a height of100 μm are formed on a glass epoxy substrate in such a manner that endsurfaces of the patterns are opposed to each other. Then, the endsurface of each of the copper patterns is plated with Ag in a thicknessof 3 μm and is further plated with Sn in a thickness of 0.5 μm.

Thereafter, the glass epoxy substrate is heated to a temperature of 250°C. to cause mutual diffusion of Ag and Sn, thereby forming protectionlayers containing Ag₃Sn as a chief component. Then, a sample of Exampleis prepared by bonding the protection layers to each other using theSn-3.5 wt % Ag alloy (the solder).

FIG. 5A is a view schematically depicting the sample of Example, inwhich reference numeral 31 denotes the copper pattern, reference numeral32 denotes the protection layer (Ag₃Sn), and reference numeral 33denotes the solder (Sn-3.5 wt % Ag).

Meanwhile, as depicted in FIG. 5B, a sample similar to Example exceptthat no protection layers 32 are provided is prepared as ComparativeExample 1.

Further, as depicted in FIG. 5C, a sample similar to Example except thatNi-plated layers 34 each having a thickness of 13 μm to 16 μm are formedinstead of the protection layers 32 is prepared as Comparative Example2.

In order to achieve uniform shapes of solder bonded portions, a resistfilm is formed so as to prevent the solder from adhering to portionsother than the end surfaces of the copper patterns 31. In addition, aplurality of samples are prepared for each of Example and ComparativeExamples 1 and 2.

Next, the samples of Example and Comparative Examples 1 and 2 areimmersed in an oil bath maintained at a temperature of 160° C. in orderto reduce temperature variations due to Joule heating associated withchanges in resistance. Then, a direct current is applied from a constantcurrent regulator to the samples of Example and Comparative Examples 1and 2 in such a condition that a current density at a bonded interfacebetween the solder and the copper pattern is equal to 2.5×10⁴ A/cm².Then, time to cause a wiring disconnection due to electromigration ismeasured for each of the samples.

FIG. 6 is a graph illustrating results of investigation of the time tocause wiring disconnections in the samples of Example and ComparativeExamples 1 and 2. In this graph, the horizontal axis indicates the timeto cause a wiring disconnection (fracture lifetime) and the verticalaxis indicates a distribution function F. As apparent from FIG. 6, thefracture lifetime of the samples of Example is about three times as longas the fracture lifetime of the samples of Comparative Example 1 andabout six times as long as the fracture lifetime of the samples ofComparative Example 2. The results of this experiment proveeffectiveness of the embodiment.

In the above-described experiment, the end surfaces of the copperpatterns are sequentially plated with Ag and Sn and are then subjectedto thermal treatment to form the AgSn alloy. Nevertheless, similarresults are also achieved in the case where the end surfaces of thecopper patterns are directly plated with the AgSn alloy.

Experiment 2

An experiment is conducted for investigating relations between Agcontents in the protection layers and time to cause wiringdisconnections due to electromigration. Results are described below.

Samples similar to the sample of Example in the above-describedExperiment 1 (see FIG. 5A) are prepared by setting various Ag contentsin the protection layers covering the end surfaces of the copperpatterns. Then, a direct current at a current density of 2.5×10⁴ A/cm²is applied to each of the samples as similar to the procedures used forthe samples of Example in Experiment 1. Then, time to cause a wiringdisconnection is measured for each of the samples.

FIG. 7 is a graph illustrating results of investigation of relationsbetween the Ag contents in the AgSn alloys and the time to cause wiringdisconnections (fracture lifetime), in which the horizontal axisindicates the Ag contents in the AgSn alloys while the vertical axisindicates the time. As apparent from FIG. 7, the fracture lifetime isequal to or below 500 hours when the Ag content falls below 5 wt % orexceeds 95 wt %. The results of this experiment prove that the Agcontent in each protection layer is preferably in a range from 10 wt %to 95 wt % inclusive.

In the embodiment, the description has been given of the case where theelectronic components are the semiconductor device (an LSI) and thecircuit board. Needless to say, the techniques disclosed above may bealso applied to electronic components other than the semiconductordevice, such as a chip resistor element or a capacitor element. Inaddition, though the description has been given of the case of bondingthe semiconductor device to the circuit board in the embodiment, theembodiment may be also applied to a case of soldering semiconductordevices to each other.

All examples and conditional language recited herein are intended forthe pedagogical purposes of aiding the reader in understanding theinvention and the concepts contributed by the inventor to further theart, and are not to be construed as limitations to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority andinferiority of the invention. Although one or more embodiments of thepresent invention have been described in detail, it should be understoodthat the various changes, substitutions, and alterations could be madehereto without departing from the spirit and scope of the invention.

We claim:
 1. An electronic component comprising a connection terminal tobe soldered to a different electronic component, wherein a surface ofthe connection terminal before being soldered to the differentelectronic component is directly covered with a protection layer made ofa AgSn alloy.
 2. The electronic component according to claim 1, whereinthe AgSn alloy is Ag₃Sn.
 3. The electronic component according to claim1, wherein a Ag content in the protection layer is equal to or above 10wt % and equal to or below 95 wt %.
 4. The electronic componentaccording to claim 1, wherein the connection terminal is made of Cu. 5.An electronic device comprising: an electronic component; a circuitboard having the electronic component mounted thereon; and solderbonding a connection terminal of the electronic component to aconnection terminal of the circuit board, wherein a surface of at leastone of the connection terminal of the electronic component and theconnection terminal of the circuit board before being bonded to eachother with the solder is directly covered with a protection layer madeof a AgSn alloy.
 6. The electronic device according to claim 5, whereinthe solder is an alloy of Ag and Sn.
 7. The electronic device accordingto claim 5, wherein the AgSn alloy is Ag₃Sn.
 8. The electronic deviceaccording to claim 5, wherein a current flowing between the connectionterminal of the electronic component and the connection terminal of thecircuit board has a current density equal to or above 10⁴ A/cm².
 9. Theelectronic device according to claim 5, wherein the protection layer isprovided only on the connection terminal on a low-potential side out ofthe connection terminal of the electronic component and the connectionterminal of the circuit board.
 10. The electronic device according toclaim 5, wherein the connection terminal of the electronic component andthe connection terminal of the circuit board are made of Cu.